1. Field of the Invention
The present invention relates to a method for controlling injection in a die casting machine and apparatus for the same. More specifically, it relates to an injection control in a die casting machine for producing a high-quality die-casting product.
2. Description of Related Art
Die-Casting Machine and Pressurization!
Conventionally, it is known that a quality of a die-casting product is greatly influenced by an injection speed and an injection pressure in filling molten material in a casting mold. Especially, the molten material has to be sufficiently pressurized before the molten material solidifies. For that reason, a high-filling-injection-power type die-casting machine having an injection cylinder of large diameter is conventionally known.
However, though the above-described die-casting machine can generate a great filling injection power, the filling injection power is unstable and difficult to reproduce precisely since back pressure of the injection cylinder is controlled on meter-out side while the injection cylinder is in operation. Furthermore, the above-described die-casting machine is inferior in view of energy consumption.
Accordingly, another type of die-casting machine having two-stage driving cylinder (for injecting and boosting pressure of the molten material) device comes to be employed in recent years.
Generally, an injection plunger is advanced at a low speed to initiate filling the molten resin into a mold cavity, thereby preventing air from letting in the molten material. After the head of the molten material reaches a gate portion of the mold to increase a pressure of an injection cylinder device for filling the molten resin, the injection plunger is advanced at a high speed so that a temperature of the molten material is not lowered, thereby rapidly filling the molten material into the mold cavity.
After above-described injection process, a high pressure is applied to the injection cylinder device by a boost cylinder device to boost the pressurization to the molten material in the mold by the injection plunger (boosting pressure process) when the molten material is filled in the mold to further increase the pressure of the injection cylinder device or when the injection plunger is advanced to a predetermined position corresponding to completion of filling the molten material.
Tow-Stage Cylinder Die-Casting Machine!
The conventional two-stage cylinder die-casting machine is specifically described below.
In FIG. 10, the die-casting machine has an injection sleeve 93 into which molten material 92 to be filled into a mold cavity 91, an injection cylinder device 95 for driving the injection plunger 94 to inject the molten material 92, and a boost cylinder device 96 having a large diameter for applying high pressure to hydraulic fluid at a back side of the injection cylinder device 95 after completion of filling the molten material 92. Accordingly, a pressure applied to the molten material 92 filled in the mold cavity 91 is boosted through the injection cylinder device 95.
FIG. 11 shows an injection speed change CV and an injection pressure change CP in the injection process and the boosting pressure process of the above-described die-casting machine. In the FIG. 11, the injection cylinder device 95 is advanced at a low speed VL at the start and is advanced at a high speed VH from a time period t1. The injection cylinder advance is restrained by a filling pressure of the molten material 92 in accordance with completion of filling the mold cavity 91 and is stopped at time period t3, at which the injection cylinder device 95 reaches a stroke end thereof. During the advance of the injection cylinder device 95, the boost cylinder 96 is actuated to boost the pressurization, so that the pressure applied to the molten material in the mold cavity 91 reaches PH.
For controlling sequence (switch from the injection process to the boosting pressure process) of the injection cylinder device 95 and the boost cylinder device 96, a sequence valve method where the switch is carried out by detecting the change in the injection pressure, and limit switch method where the switch is carried out by detecting the advanced position of the injection plunger are adopted.
In the above limit switch method, following hydraulic circuit is employed.
In FIG. 12, the injection cylinder device 95 has connected thereto an injection hydraulic circuit 114 including a check valve 111, an injection speed controlling valve 112 and an accumulator 113. The boost cylinder device 96 has connected thereto a boost hydraulic circuit 117 including a pilot-operation boost control valve 116 which is opened and shut by an electric switching valve 115 and an accumulator 113.
The electric switching valve 115 is designed to open the boost control valve 116 when the pressure of the injection hydraulic circuit 114 exceeds a predetermined boost starting pressure. Accordingly, when the injection cylinder device 95 starts advancing by operating the injection speed control valve 112 to inject the molten material 92 and the filling pressure is increased in accordance with completion of the molten material into the mold to reach the predetermined boost starting pressure, the electric switching valve 115 is operated to open the boost control valve 116, thereby initiating advance of the boost cylinder device 96 to boost pressurization. A pilot check valve 97 is connected to a meter-out side of the boost cylinder device 96. The pilot check valve 97 is opened before boosting pressurization to lessen a hydraulic resistance of the back-pressure side of the boost cylinder device 96, and is shut after the injection cylinder device 95 is retreated.
FIG. 13 specifically shows the injection cylinder device 95 and the boost cylinder device 96.
The injection cylinder device 95 has an injecting piston 95A thereinside, and the injecting piston 95A is advanced by a hydraulic pressure of hydraulic fluid supplied to a back side of the injecting piston 95A by the injection hydraulic circuit 114. The flow of the hydraulic fluid of the injection hydraulic circuit 114 is controlled by the injection speed control valve 112, thereby switching the advance and halt of the injecting piston 95A and controlling advance speed of the injecting piston 95A.
The boost cylinder device 96 has an boosting piston 96A thereinside, and the boosting piston 96A is advanced by a hydraulic pressure of a hydraulic pressure of hydraulic fluid supplied to a back side of the boosting piston 96A by the boost hydraulic circuit 117, thereby pressurizing the injecting piston 95A from the back side thereof through a intermediate member 95B of the injection cylinder device 95. The hydraulic oil from the boost hydraulic circuit 117 is controlled to flow on and off by the boost control valve 116, thereby switching advance and halt of the boosting piston 96A.
The boost control valve 116 is opened and shut by the electric switching valve 115. A solenoid valve and the like switching in accordance with filling pressure are employed as the electric switching valve 115.
Problem in the Conventional Two-Stage Die-Casting Machine!
Incidentally, in the above-described conventional two-stage die-casting machine, there is a disadvantage that the injection speed is not stable in injecting a highly viscous molten material such as a semi-solid molten material, thereby deteriorating the quality of the die-casting product. The disadvantage is described below with reference to FIG. 14.
The FIG. 14 shows an injection speed change CV and an injection pressure change CP in injecting highly viscous molten material.
In the FIG. 14, the injecting piston of the injection cylinder device is smoothly advanced at a determined speed of 0.7 m/s until the head of the molten material reaches around entrance of the gate portion of the mold. However, a great change in gate resistance is caused in injecting the molten material into the mold cavity by the boost cylinder device during a stroke shown as S, since a high injection power is necessary after the head of the molten material passes the entrance of the gate portion.
Accordingly, an irregular fluctuation shown as Pp is caused in casting pressure, and an inappropriate change shown as Vp, which is not predetermined, is occurred in the injection speed. The quality of the diecasting product is influenced by the change in the injection speed.